1. Field of the Invention
The present invention relates to a magnetic ferrite microwave resonator and more particularly to a magnetic ferrite microwave resonator including a magnet to bias a ferrite in the resonator so that the resonator is sensitive to changes in an applied magnetic field to provide tunability.
2. Discussion of the Related Art
Microwave resonators are frequently used in narrow band filter applications. These resonator structures can include superconductive materials and have a resonant frequency and quality factor fixed by the geometry of the resonator and the intrinsic microwave impedance of the elements that make up the resonator. Generally, a resonator receives a signal and only allows the portion of the signal at a specific frequency, the resonant frequency, to pass. Different applications of the resonator frequently require that different frequencies be passed. Therefore, some frequency tunability of the resonant frequency is desired.
Tunability may be achieved by providing a ferroelectric material near the resonator and adjusting a voltage applied to the resonator to bias ferroelectrics in the resonator. Some devices currently in use, apply an electric field directly to the ferroelectrics to adjust the permittivity of ferroelectric materials in the vicinity of the resonant structure. Ferroelectric materials, however, have intrinsically broad microwave losses and can severely degrade the performance of high quality resonators.
Efficient filter resonator structures have a high Q value, which is the electrical gain/loss ratio (Q) equal to the resonant frequency (vc) over a change in frequency (xcex94v) as shown in the graph of FIG. 1.
U.S. Pat. No. 4,887,052, entitled xe2x80x9cTuned Oscillator Utilizing Thin Film Ferromagnetic Resonator,xe2x80x9d by Murakami et al., discloses a resonator including a microstrip structure in which the signal line is formed of YIG, a ferromagnetic material, spaced from a ground plane. Thus, the YIG film actually forms part of the resonator microstrip structure and the center frequency of the resonator equal to the ferromagnetic resonance frequency of the YIG film.
In accordance with the present invention, certain disadvantages of conventional apparatuses are resolved by having an electromagnetic filter comprising a resonator portion with an input for receiving an electromagnetic signal and an output for emitting an electromagnetic signal. A tuning portion is further provided including a magnetic ferrite element coupled to the resonator disposed in first and second magnetic fields generated by a fixed magnet and an electromagnet. Thus the magnetic ferrite element has a magnetic permeability determined by the first and second magnetic fields. Specifically, the first magnetic field places a ferromagnetic resonance frequency of the ferrite element near a frequency of the electromagnetic signal transmitted by the resonator portion. The second magnetic field is variable in response to a varying current supplied to the electromagnet to change the permeability of the ferrite element, to thereby alter the center frequency (Vc) of the resonator, thereby facilitating tuning of the electromagnetic signal.
In another embodiment, a bandpass filter includes a plurality of filters connected in parallel where each filter includes a transmission line for transmitting electromagnetic radiation, and a tuning portion that further includes a ferrite element, a permanent magnet for generating a first magnetic field, and an electromagnet for generating a second magnetic field. The ferrite element is disposed in the first and second magnetic fields such that the first magnetic field places a ferromagnetic resonance frequency of the ferrite element near a frequency of the electromagnetic radiation transmitted by the transmission line. The second magnetic field is variable in response to a varying current supplied to the electromagnet to change the permeability of the ferrite element so as to modulate the center frequency and facilitate tuning.
In another embodiment of the present invention, a method is provided for tuning a filter, where the filter includes a ferrite element disposed adjacent a transmission line, an electromagnet, and a permanent magnet. The method includes the steps of generating a magnetic field using the electromagnet, subjecting the ferrite element to the magnetic field generated by the electromagnet, and varying the field generated by the electromagnet to change a magnetic permeability in the ferrite element to modulate the electromagnetic signal carried by the transmission line.